1. Field of the Invention
The present invention relates to a method for manufacturing an electrode plate for a battery. More specifically, the present invention relates to a method for manufacturing an electrode plate for a battery that can reduce the short-circuit failure between electrode plates after incorporation into a battery.
2. Description of the Related Art
In recent years, global warming and the exhaustion of resources, such as fossil fuel, have been serious problems. To deal with these problems, electric vehicles, including pure electric vehicles powered by a battery alone and hybrid electric vehicles powered by a battery and gasoline, have begun to spread and take the place of gasoline-powered vehicles. At present, a nickel metal-hydride storage battery is used as a driving power source for these electric vehicles. The market for the electric vehicles employing such a nickel metal-hydride storage battery or lithium ion secondary battery as a driving power source is expected to expand in the future.
An electrode plate used in these batteries is provided by cutting a flat electrode sheet to a predetermined size. The following is a conventional method for cutting an electrode plate for a battery: as shown in FIGS. 4 and 5, the entire regions on both sides of a portion to be cut 4 of an electrode sheet 3 are each fixed between a pair of upper and lower flat cutting dies 6 having the same area as an electrode plate; two adjacent pairs of upper and lower cutting dies 6 on both sides of the portion to be cut 4 are moved perpendicularly to the electrode sheet 3 in opposite directions (upward and downward), and thus the electrode sheet 3 is cut to a predetermined size. Moreover, to suppress burrs that are generated on the electrode plate after cutting, a method for reducing the thickness of the portion of the electrode sheet to be cut has been proposed as well.
However, the above conventional cutting method causes the adhesion of foreign materials to the electrode plate surface and an edge burr, which are largely responsible for the short-circuit failure between electrode plates after incorporation into a battery. The adhesion of foreign materials to the electrode plate surface occurs because fragments are pressed on the electrode plate surface by the cutting dies when the electrode sheet is cut with the adjacent flat cutting dies being in contact with the entire regions on both sides of the portion to be cut. Also, the edge burr is generated when the flat electrode sheet is cut. Moreover, even if the portion of the electrode sheet to be cut is thin, the edge burr occurs because the thin portion is bent during cutting and the burrs are not suppressed completely.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a method for manufacturing an electrode plate for a battery that can reduce the short-circuit failure between electrode plates after incorporation into a battery by suppressing the adhesion of fragments to the electrode plate surface and the generation of burrs.
To achieve the above object, a method for manufacturing an electrode plate for a battery of the present invention includes cutting an electrode sheet containing an active material to a predetermined size with cutting dies. The regions on both sides of and close to a portion of the electrode sheet to be cut are each fixed between a pair of cutting dies. Two adjacent pairs of cutting dies on both sides of the portion to be cut are moved perpendicularly to the electrode sheet in opposite directions to each other, and thus the electrode sheet is cut. This method can lower the frequency with which fragments produced in cutting the electrode sheet are pressed on the electrode plate surface by the cutting dies, thus reducing the adhesion of foreign materials to the electrode plate surface. As a result, the short-circuit failure between electrode plates can be reduced after the electrode group produced in the above manner has been incorporated into a battery. Here, xe2x80x9cthe region close to a portion to be cutxe2x80x9d, in this context, refers to the region within about one-tenth of the width and one-twentieth of the length of the electrode plate cut (hereinafter, used in the same way).
In the above method of the present invention, it is preferable that the portion of the electrode sheet to be cut and the regions on both sides of and close to that portion are formed previously so as to have a thickness smaller than that of the electrode sheet body, and when the electrode sheet is cut, the thin regions on both sides of the portion to be cut are each fixed between a pair of cutting dies. This preferred example can reduce the adhesion of foreign materials on the electrode plate surface and prevent burrs from being generated on the electrode surface as well. The reason for this is as follows: since a three-dimensional porous metal substrate is in sponge form, it is hardened by reducing the thickness; in addition, a surface burr (the height of a burr from the electrode plate surface) tends to be higher with increasing thickness of the substrate. Thus, the short-circuit failure between electrode plates can be reduced further after the electrode group produced in the above manner has been incorporated into a battery. In this case, it is preferable that the thickness of the thin region is one-half to one-fifth of that of the electrode sheet body. Though the burrs are not generated readily with reducing the thickness of the substrate, an excessive reduction in thickness makes cutting difficult. Moreover, in this case, it is preferable that the portion of the electrode sheet to be cut and the regions on both sides of and close to that portion are formed to be thin by pressing.
In the above method of the present invention, it is preferable that the electrode sheet is formed by filling a substrate of three-dimensional porous metal body with the active material.
In the above method of the present invention, it is preferable that the electrode sheet is formed by filling a substrate of three-dimensional porous metal body with the active material, and that the regions on both sides of and close to the portion of the electrode sheet to be cut are coated with resin or impregnated with a liquid containing a resin component before cutting the electrode sheet. This preferred example can reduce the adhesion of foreign materials to the electrode plate surface and prevent burrs from being generated on the electrode plate surface as well. Moreover, in this case, it is preferable that the method includes the following steps in no special order before cutting the electrode sheet: a step of coating the regions on both sides of and close to the portion of the electrode sheet to be cut with resin or a step of impregnating the regions with a liquid containing a resin component, and a step of forming the portion of the electrode sheet to be cut and the regions on both sides of and close to that portion so as to have a thickness smaller than that of the electrode sheet body. This preferred example further can reduce the adhesion of foreign materials to the electrode plate surface and prevent burrs from being generated on the electrode plate surface as well.
In the above method of the present invention, it is preferable that the electrode sheet is formed by applying the active material to a core material of punching metal, that a region to which no active material is applied is provided previously in the portion of the electrode sheet to be cut and the regions on both sides of and close to that portion, and when the electrode sheet is cut, the regions with no active material on both sides of the portion to be cut are each fixed between a pair of cutting dies. This preferred example can reduce the adhesion of foreign materials to the electrode plate surface. In addition, since a plain region is cut, burrs generated on the electrode plate surface can be prevented as well. Thus, the short-circuit failure between electrode plates further can be reduced after the electrode group produced in the above manner has been incorporated into a battery.
As described above, the present invention can prevent fragments of the electrode sheet from being pressed on the electrode plate surface to form foreign materials thereon and burrs from being generated on the electrode plate surface. Thus, the short circuits of the electrode group for a battery can be suppressed.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.